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Abstract:

The invention provides a light emitting diode device comprising a light
emitting diode arranged on a substrate and a wavelength converting
element. The wavelength converting element contains a luminescent
material a Mn4+-activated fluoride compound having a garnet-type
crystal structure. The Mn4+-activated fluoride compound preferably
answers the general formula {A3}[B2-x-yMnxMgy]
(Li3) F12-dOd, in which formula A stands for at least
one element selected from the series consisting of Na.sup.+ and K.sup.+
and B stands for at least one element selected from the series consisting
of Al3+, B3+, Sc3+, Fe3+, Cr3+, Ti4+ and
In3+, and in which formula x ranges between 0.02 and 0.2, y ranges
between 0.0 (and incl. 0.0) and 0.4 and d ranges between 0 (and incl. 0)
and 1. As the luminescent materials of the described type and structure
have high stability and low sensitivity towards humid environments, they
can advantageously be used as in wavelength conversion elements of LED
devices.

Claims:

1. A light emitting diode device comprising: a light emitting diode
arranged on a substrate, and a wavelength converting element containing a
Mn4+-activated fluoride compound as a luminescent material, wherein
the Mn4+-activated fluoride compound has a garnet-type crystal
structure.

2. A light emitting diode device according to claim 1, wherein the
Mn4+-activated fluoride compound answers the formula
{A3}[B2-x-yMnxMgy](Li3) F12-dOd, in
which formula A stands for at least one element selected from the series
consisting of Na.sup.+ and K.sup.+ and B stands for at least one element
selected from the series consisting of Al.sub.3.sup.+, B.sub.3.sup.+,
Sc.sub.3.sup.+, Fe.sub.3.sup.+, Cr.sub.3.sup.+, Ti.sub.4.sup.+ and
In.sub.3.sup.+, and in which formula 0.02<x<0.2,
0.0.ltoreq.y<0.4, and 0.ltoreq.d<1.

4. A light emitting diode device according to claim 1, wherein the
wavelength converting element is formed as a ceramic platelet.

5. A light emitting diode device according to claim 1, wherein the
wavelength converting element is formed as a shaped body of resin
material in which an amount of the Mn4+-activated fluoride compound
is incorporated.

7. A luminescent material according to claim 6, wherein the composition
answers the formula {A3}[B2-x-yMnxMgy](Li3)
F12-dOd, in which formula A stands for at least one element
selected from the series consisting of Na.sup.+ and K.sup.+ and B stands
for at least one element selected from the series consisting of
Al.sub.3.sup.+, B.sub.3.sup.+, Sc.sub.3.sup.+, Fe.sub.3.sup.+,
Cr.sub.3.sup.+, Ti.sub.4.sup.+ and In.sub.3.sup.+, and in which formula
0.02<x<0.2, 0.0.ltoreq.y<0.4, and 0.ltoreq.d<1.

8. A luminescent material according to claim 7, wherein the composition
of the Mn4+-activated fluoride compound substantially answers the
formula {A3
}[B2-x-yMnxMgy](Li3)F12-dOd.

9. A method for preparing a luminescent material having a composition
according to claim 6, characterized in that it encompasses the following
steps: Preparing a first aqueous solution by dissolving K2MnF6
in water containing at least 20 vol % HF, Preparing a second aqueous
solution of salts of the remaining metals of which the intended garnet is
composed, in molar ratio's corresponding to the garnet composition,
Mixing stoichiometric amounts of both solution while stirring the
resulting mixture, and Isolating the resulting garnet composition from
the mixture.

10. A method for preparation a luminescent material according to claim 8,
wherein the first aqueous solution contains NaHF.sub.2.

Description:

BACKGROUND OF THE INVENTION

[0001] The invention relates to a light emitting diode device comprising a
light emitting diode arranged on a substrate, and a wavelength converting
element containing a Mn4+-activated fluoride compound. The invention
also relates to a luminescent material as well as to a method for
preparing such luminescent material.

[0002] Light emitting diode devices (abbr. as LED devices) are widely
known as new semiconductor light sources with promising lighting
properties for future applications. These LED devices should eventually
substitute many of the current light sources, like incandescent lamps.
They are especially useful in display lights, warning lights, indicator
lights and decoration lights.

[0003] The color of the emitted light depends on the type of semiconductor
material. LEDs produced from Group III-V alloys--such as GaN--are
well-known for their ability to produce emission in the green to UV range
of the electromagnetic spectrum. During the last decade, methods have
been developed to convert (parts of) the radiation emitted by such `blue`
or `(near)UV` LEDs into radiation of longer wavelength. Phosphors are
widely used luminescent materials for this purpose. These phosphors are
crystalline, inorganic compounds of high chemical purity and precisely
controlled compositions. They comprise small amounts of specifically
selected elements (`activators`), which make them to efficient
luminescent materials.

[0004] In addition to colored LEDs, the development of so-called `white
light LEDs` is also very important. An interesting configuration in this
field is based on converting a part of the light generated by a blue/UV
LED and mixing that converted part with the non-converted part of said
generated light, so obtaining white or white-like light. In this area
blue emitting GaInN LEDs are most popular. Ce3+-activated Yttrium
Aluminum Garnet (YAG-Ce) and Eu2+-activated Ortho Silicates (BOSE,
OSE) are well-known phosphors for this purpose.

[0005] A LED device as described in the opening paragraph is known as
such, for example from the patent publication WO 2009/012301-A2. This
document describes in great detail a number of LED devices in which
Mn4+-activated fluoride compounds are applied as a luminescent
material in the wavelength converting elements of these devices. Emission
and excitation spectra of a number of K2[XF6]:Mn4+(X=Nb or
Ta) and K3[XF7]:Mn4+ (X=Bi, Y, La or Gd) phosphor
compounds are shown. These luminescent materials appear to show a narrow
band or line emission in the red spectral region (600-660 nm) of the
electromagnetic spectrum. This is very attractive as LED devices
comprising such luminescent materials in their wavelength converting
elements are able to produce `warm white` light. This is light with a
comparative color temperature (CTT) below 5000K.

OBJECTS AND SUMMARY OF THE INVENTION

[0006] The known LED devices have several disadvantages. A first
disadvantage to be mentioned concerns the fluoride compounds used in the
wavelength converting elements, most of which are (less or more) toxic. A
second disadvantage pertains to the handling of these fluoride compounds,
which in practice is not easy, due to their relatively high sensitivity
towards humid environments. Prolonged exposure of these materials to
(humid) air causes formation of a thin water film on the surface of the
material, leading to (surface) decomposition. This disadvantageous
property affects both the pure materials (causing short shelf times) and
the LED devices in which they are applied (causing decrease of
performance in time).

[0007] The current invention aims at circumventing at least the mentioned
drawbacks of the known devices.

[0008] In addition, the invention has as an object to provide new LED
devices with wavelength converting elements containing
Mn4+-activated fluoride compounds which are less toxic and less
sensitive towards humid environments.

[0009] A further object is providing a novel class of Mn4+-activated
fluoride compounds with attractive luminescent properties for use in LED
devices, which should preferably provide the devices the possibility of
producing warm white light.

[0010] According to the present invention, these and other objects are
achieved by providing a light emitting diode device comprising a light
emitting diode arranged on a substrate and a wavelength converting
element containing a Mn4+-activated fluoride compound as a
luminescent material, wherein the Mn4+-activated fluoride compound
has a garnet-type crystal structure.

[0011] The invention is based on the insight gained by the inventors that
the sensitivity towards humid environments of Mn4+-activated
fluoride compound with a garnet-type crystal structure is considerably
less than the sensitivity towards humid environment of the known
compounds described in WO 2009/012301. The described compounds do not
have a garnet-type crystal structure. The inventors moreover believe
that, in view of the chemically inert character of the new invented
luminescent compounds, their toxicity is low as compared with similar
known compounds disclosed in said patent publication. These properties of
the luminescent compounds make their application in LED devices more
attractive, both in the production and in the use of the devices.

[0012] Fluorine compounds having a garnet-type crystal structure can be
represented by the following general formula:
{A3}[B2](C3)F12, in which F stands for fluoride and
in which A, B and C represent ions of metal or metal-like elements. These
three types of ions are positioned respectively on the dodecahedral, the
octahedral and the tetrahedral sites of the garnet crystal structure.
Generally speaking, elements A and C are monovalent (+) whereas element B
is trivalent (3+). However, especially on the octahedral sites,
substitutions with charge compensations are possible, so that also
combinations of a bivalent and a tetravalent metal ion on these sites can
be found.

[0013] The presence of both Mn4+ and F.sup.- ions in the garnet
structure is believed to be essential for providing the interesting
narrow band or line emission in the red spectral region of the
electromagnetic spectrum. This means the region between appr. 600 and
appr. 660 nm.

[0014] A preferred embodiment of the LED device according to the present
invention is characterized in that the Mn4+-activated fluoride
phosphor compound answers the formula {A3}
[B2-x-yMnxMgy](Li3)F12-dOd, in which
formula A stands for at least one element selected from the series
consisting of Na.sup.+ and K.sup.+ and B stands for at least one element
selected from the series consisting of Al3+, B3+, Sc3+,
Fe3+, Cr3+, Ti4+and In3+, and in which formula x
ranges between 0.02 and 0.2, y ranges between 0.0 (and incl. 0.0) and 0.4
(i.e. 0.0≦y<0.4) and d ranges between 0 (and incl. 0) and 1
(i.e. 0≦d<1).

[0015] Although the above-mentioned inventive insight in principle can be
achieved with all possible Mn4+-activated fluoride compounds having
a garnet-type crystal structure, especially compounds with Na.sup.+
and/or K.sup.+ on the dodecahedral sites, Al3+, B3+, Sc3+,
Fe3+, Cr3+, Ti4+and/or In3+ on the octahedral sites
and Li.sup.+ on the tetrahedral sites are preferred. Based on ion-radii
considerations in combination with requirements posed by the spatial
structure of garnets, these preferred compounds are believed to form
highly stable crystalline compounds.

[0016] The Mn4+ ions are believed to be located on octahedral sites
of the garnet crystal structure. Ion radii calculations show that
Mg2+ is preferably present on the same crystal sites for charge
compensation reasons. The amount of Mn4+ in the preferred compounds
ranges between 1 and 10 mol % based on the total B3+-ion content. A
higher amount of Mn4+ ions appears to cause a high so-called `self
quenching`. If less than 1 mol % Mn4/ is present on the octahedral
sites of the garnet structure, no or hardly any activating effect is seen
in the LED device. In such materials, the absorption on Mn4+ appears
to be negligible. Mn4+-amounts between 5 and 8 mol % are preferred,
as in these conditions an optimal match between both the self-quenching
effect and the desired absorbance level is reached.

[0017] In the preferred embodiment of the LED device, Mg2+ is also
present on the octahedral sites in the garnet structure. The presence of
Mn4+ causes charge imbalance in the garnet structure, which can be
compensated by the presence of Mg2+. The amount of Mg2+ can be
chosen somewhat broader as the amount of Mn4+. Therefore the amount
of Mg2+ in the preferred garnet compounds may range between 0 and 20
mol % based on the total B3+-ion content, whereby the range includes
the value 0 mol %. A higher amount of Mg2+ ions appears to cause the
negative effect of lattice defects, e.g. anion vacancies.
Mg2+-amounts between 1 and 10 mol % are preferred, as in these
conditions an optimal match between both charge compensation and
luminescence efficiency is reached.

[0018] Practice has shown that the amount of F.sup.- can somewhat deviate
from the stoichiometrical amount of 12 atoms per crystal cell unit. This
deviation is indicated by the factor d, which ranges between 0 (and incl.
0) and 1. It is stressed that, due to charge compensation effects, a
small amount of the F.sup.- can also be replaced by oxygen. This can be
the case if a small part of the trivalent ions of the octahedral sites
are replaced by ions of higher valence, like Ti4+. Under usual
conditions, this will always be below appr. 8 mol % and is preferably
below 4 mol %, all based on the total amount of F.sup.- in the garnet
structure. An increase in the amount of O2- at the expense of
F.sup.- in the garnet structure most generally causes an increased shift
of the emission of the phosphor compound into the deeper red, which is
undesired.

[0019] A more preferred embodiment of the LED device according to the
present invention is characterized in that in that the composition of the
Mn4+-activated fluoride compound substantially answers the formula
{Na3}[Al2-x-yMnxMgy](Li3)F12-dOd. The
ranges of the indices are as described before. From experimental data, it
was concluded that, within the described broader class of garnet-type
compounds, this series of compounds is extremely stable. This stability
makes the application of these compounds in LED devices very attractive,
both in the production and in the use of the devices.

[0020] A further interesting embodiment of the LED device according to the
invention has the feature that the wavelength converting element is
formed as a ceramic platelet. This feature has especially value in LED
devices to be used for producing white light. In principle the
luminescent material can be formed with or without additional filler
materials by pressing the materials to a sheet, sintering these sheets
according to a certain heating procedure and separating platelets of
desired dimensions from said sintered sheet, for example by (laser)
carving and breaking. As in this manner ceramic platelets of precise
thickness can be manufactured, wavelength converting elements formed of
such platelets are very suitable in LED devices which should convert
(ncar)UV or blue LED light into white light.

[0021] Another interesting embodiment of the LED device according to the
invention has the feature that the wavelength converting element is
formed as a shaped body of resin material in which an amount of the
Mn4+-activated fluoride compound is incorporated. Said shaped body
can for example be formed as a lens or as a plate. However, other
structures are also possible within the scope of the invention. The
amount of fluoride compound with garnet-type crystal structure in the
resin can be chosen dependent on the desired amount of converted light,
the volume of the body, etc.

[0022] The invention also provides a new luminescent material containing a
Mn4+-activated fluoride compound. This material is characterized in
that the compound has a garnet-type crystal structure. Materials of this
composition are relatively less toxic, have relatively low sensitivity
towards humid environments and show interesting emission spectra in the
near red region of the electromagnetic spectrum (600-660 nm).

[0023] Especially interesting is the material that answers the formula
(A3) [B2-x-yMnxMgy](Li3)F12-dOd, in
which formula A stands for at least one element selected from the series
consisting of Na.sup.+ and K.sup.+ and B stands for at least one element
selected from the series consisting of Al3+, B3+, Sc3+,
Fe3+, Cr3+, Ti4+ and In3+, and in which formula x
ranges between 0.02 and 0.2, y ranges between 0.0 (and incl. 0) and 0.4
and d ranges between 0 (and incl. 0) and 1. This material can be
advantageously applied in phosphor-coated LED devices. This holds
especially for the luminescent material the composition of which
substantially answers the formula
{Na3}[Al2-x-yMnxMgy](Li3)F12-dOd. For
reasons described before, luminescent materials wherein the amount of
Mn4+ is between 1 and 10 mol % whereas the amount of Mg2+ is
between 1 and 20 mol % are preferred. Most preferred however are
compositions with a Mn4+-content between 5 and 8.0 mol % and an
Mg2+-content between 1 and 10 mol %.

[0024] Another interesting aspect of the invention relates to a method for
preparing a luminescent material as described in the previous paragraph.
This method is characterized in that it encompasses the following steps:
[0025] Preparing a first aqueous solution by dissolving K2MnF6
in water containing at least 20 vol % HF, [0026] Preparing a second
aqueous solution of salts of the remaining metals of which the intended
garnet is composed, in molar ratio's corresponding to the garnet
composition, [0027] Mixing stoichiometric amounts of both solution while
stirring the resulting mixture, and [0028] Isolating the resulting garnet
composition from the mixture.

[0029] It will be clear to the skilled persons that the sequence in which
the first and second aqueous solution are prepared is of no importance.
It is however highly preferred that, during the mixing of these
solutions, the second solution is added to the first solution during
stirring the so formed mixture. Care should be taken that the amount of
added second solution is chosen so that a stoichiometric amount of
Mn4+ to the amount of the other metals, which are already in
stoichiometric amounts available in the first solution.

[0030] It is preferred that the first aqueous solution contains a small
amount of NaHF2. Adding this compound prevents that part of the
Mn4+ is reduced. After mixing the solutions and stirring the mixture
for some 5 minutes, the resulting turbid solution is filtered off and
washed several times with 2-propanol. The obtained powder is subsequently
dried under vacuum at 110° C. In order to obtain the right grain
size, the powder may be mechanically ground in a mortar. The so-obtained
powder is analyzed by X-ray and further used in wavelength converting
elements of LED devices according to the present invention.

[0031] It is stressed that not only the invented Mn4+-activated
fluoride compounds with garnet-type crystal structure in their pure form
enhance the desired performance of the light in a LED device, but that
also composite materials and mixed crystals of the invented compounds
were found to do so. Composites are defined as consisting of two or more
on finite scale distinguishable materials, e.g. core shell materials,
composite ceramics or coated particles. A mixed crystal in contrast has a
homogenous distribution of the constituting elements on atomic scale.

[0032] This invention therefore also pertains to composites of
{A3}[B2-x-yMnxMgy](Li3)F12-dOd type garnets
with oxide garnets A3B2(CO4)3 including but not
limited to YAG (Y3Al5O12),
Mg3Al2Si3O12 or Ca3Al2Si3O12.
These composites are preferably oxide garnet coatings on
{Na3}[Al2-x-yMnxMgy](Li3)F12-dOd type
phosphor particles, or core shell materials, where the
{Na3}[Al2-x-yMnxMgy](Li3)F12-dOd type
is surrounded by an oxide garnet shell. The difference between a coating
and a shell is mainly the relative amount of the respective materials,
whereas a coating is less than 10% w/w of the total material, in a core
shell material the shell may be 50% w/w or even more. The advantage of
such coated or core shell materials are the increased stability with
respect to humidity and the option to vary the refractive index of the
phosphor. With increased stability it is also expected that toxicity will
be further reduced.

[0033] The same advantages are expected for mixed crystals of
{Na3}[Al2-x-yMnxMgy](Li3)F12-dOd with
oxide garnets, with the general formula of the mixed crystals: (1-a)
{Na3}[Al2-x-yMnxMgy](Li3)F12-dOd*a
A3B2(CO4)3. The formation of such mixed crystals was
found to enable the variation of excitation and emission wavelengths
maxima and influence thermal quenching properties.

BRIEF DESCRIPTION OF THE DRAWING

[0034] The invention will be explained and illustrated in terms of a
number of embodiments, with the help of the drawing, in which

[0035] FIG. 1 shows a first embodiment of the LED device according to the
invention,

[0036]FIG. 2 shows a second embodiment of the LED device according to the
invention,

[0037]FIG. 3 shows a graph of the emission spectrum of the first
embodiment according to the invention,

[0038]FIG. 4 shows a graph of the emission spectrum of the second
embodiment according to the invention, and

[0039]FIG. 5 shows a graph of the x-ray pattern of a sample of the
invented compound
{Na3}[Al1.94Mn0.03Mg0.03](Li3)F12 having a
garnet-type crystal structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0040] A first embodiment of the present invention is schematically
illustrated by FIG. 1. This Figure shows a cross-section of a LED device
comprising a semiconductor light emitting diode (1), which is connected
to a substrate (2), sometimes referred to as sub-mount. The diode (1) and
substrate (2) are connected by means of appropriate connecting means,
like solder or (metal-filled) adhesive.

[0041] The diode (1) is of the GaInN type, emitting during operation light
having a wavelength of 450 nm. In the present embodiment, said light
exits LED (1) via emitting surface (4). A wavelength converting element
(3) formed as a convex lens shaped body is positioned adjacent to LED
(1). This lens is largely made of a high temperature resistant silicone
resin, in which grains are incorporated of a Mn4+-activated fluoride
compound having a garnet-type crystal structure. Latter compound acts as
a luminescent material in the lens. In the present embodiment said
silicone resin contains 16 vol %
{Na3}[Al1.94Mn0.03Mg0.03](Li3)F12, having a
grain size of appr. 10 micron. The type of silicone is chosen so that its
refractive index is almost identical with the refractive index of the
phosphor compound, namely 1.34. By using (almost) identical refractive
indices, scattering losses of the LED light through the wavelength
converting element (3) are as low as possible.

[0042] In an alternative embodiment, the invented luminescent material is
compounded with highly transparent fluoroplastics (e.g. 3M Dyneon®
THV2030G or THV220) with matched refractive index. The resulting
composite may be transferred into a suitable shape by known techniques.
These shapes may be used as functional optical parts of the LED or simply
as components for color conversion only.

[0043] The amount of luminescent compound and the dimensions of the
wavelength converting element (3) are chosen so that all the blue light
generated by the LED (1) is converted into red light having a wavelength
of appr. 630 nm. A typical emission spectrum of the light exiting the
here described LED device is shown in FIG. 3. In this Figure, the
intensity of the emission I (arbitrary units) is measured as a function
of the wavelength λ (nm). It is stressed that, for adapting the
color of the exiting red LED light, additional phosphors of other (known)
types can be used. Thus, the invention is not limited to LED devices
comprising only a single phosphor of garnet-type crystal structure in the
wavelength converting element (3), but mixtures of this phosphor with
other (known) phosphors can be applied as well.

[0044]FIG. 2 depicts a schematic cross-section of a second embodiment of
the present invention designed as a white light generating LED device.
This Figure shows a conventional blue or (near)UV generating light
emitting diode (11), which is attached to a substrate (12) using solder
bumps (not shown). Substrate (12) has metal contact pads on its surface
to which LED (11) is electrically connected (not shown). By means of
these solder pads, LED (11) can be connected to a power supply. In the
present example, LED (11) is of the AlInGaN type and emits blue light
having a peak wavelength of appr. 420-470 nm. It goes without saying that
other semiconductor materials having other peak wavelengths can be used
as well within the scope of the present invention.

[0045] Two wavelength converting elements formed as ceramic platelets (13)
and (14) are positioned adjacent to LED (11). The platelets (13, 14) and
LED (11) can mutually be affixed by means of an adhesive (like a high
temperature resistant silicone material or a low melting glass) or by
means of mechanical clamping. In the present embodiment, an adhesive is
used. To keep unwanted absorptions as low as possible, the adhesive
layers between LED (11) and element (13) as well as between element (13)
and element (14) have been made as thin as possible.

[0046] In the present embodiment, element (13) is shaped as a red phosphor
plate whereas element (14) is shaped as a yellow phosphor plate. The
surface dimensions of both plates are almost the same as the surface
dimension of the light emitting surface (15) of LED (11), although they
may be somewhat larger without having significant effect on the (white)
exiting light. In case LED (11) is small enough, side emission of the
blue radiation from the LED (11) can be ignored. The thicknesses of both
elements are typically in the range of 50-300 micron. The actual
thickness of the platelets of course depends on the spectral power
distribution of the LED light and the type of phosphor compound present
in the platelets.

[0047] In the described embodiment, the red phosphor platelet of element
(13) was prepared of a pure Mn4+-activated fluoride phosphor
compound with a garnet-type crystal structure. For this purpose the
phosphor compound substantially answered the formula
{Na3}[Al1.94Mn0.03Mg0.03](Li3)F12 having a
garnet-type crystal structure. For the yellow phosphor platelet of
element (14), the compound Y3Al5O12:Ce (`Ce-doped YAG`)
was used.

[0048] On the LED (11) and both wavelength conversion elements (13, 14) an
optical element (16) in the form of lens structure is placed, allowing
optimization of the emission pattern of the LED device. By means of a
proper choice of this optical element, a Lambertian pattern can be
obtained, but also a pattern that allows a good coupling with an optical
waveguide structure. It is also possible to design the optical element
(16) in such a way that a uniform illumination distribution of the
generated white light is obtained. This makes the present LED device very
suitable for backlighting in LCD type applications.

[0049]FIG. 4 shows a typical emission spectrum of the described white
light generating LED device according to FIG. 2. In this Figure, the
intensity of the emission I (arbitrary units) is measured as a function
of the wavelength λ (nm). The spectrum shows emission in the red
spectral region from appr. 600--appr. 660 nm, with an emission maximum
around 630 nm.

[0050] The luminescent material used in the wavelength converting element
(3, 13) of the LED devices as described above substantially answers the
formula {Na3}[Al1.94Mn0.03Mg0.03](Li3)F12
and has a garnet-type crystal structure . Said material was obtained as
co-precipitates at room temperature from aqueous HF solution containing
Mn4+ as a dopant. For the preparation of said
(Na3)[Al1.94Mn0.03Mg0.03](Li3)F12,
stoichiometric amounts of the starting materials NaCl, LiCl,
MgCl2*6H2O and AlCl3*6 H2O as well as a small amount
of NaHF2 were dissolved in water and subsequently added to a 48% HF
aqueous solution containing K2MnF6.The concentration of
Mn4+ in the HF solution was 1 mol. %. The precipitates were
filtered, washed repeatedly with 2-propanol, and then dried at
110° C. in vacuum. The obtained product was ground in a mortar.

[0051]FIG. 5 shows an X-ray powder pattern spectrum measured on a
representative sample of one of the precipitates, using Cu-Kα
radiation. In this Figure, the number of counts (N) is shown as a
function of the diffracted angle 2Theta. With this measurement, these
samples could be identified to be
{Na3}[Al1.94Mn0.03Mg0.03](Li3)F12 having a
garnet-type crystal structure. No extra phases were detected in this
sample.

[0052] It is stressed that it is possible to use a variety of other
starting materials to produce the inventive garnet-type fluoride
phosphors via co-precipitation from aqueous solution. Especially
hydroxides, nitrates, alkoxides, and carbonates are other good starting
materials for use in the co-precipitation method. Also other metal ion
salts can be used as starting material, like with salts of K.sup.+,
B3+, SC3+, Fe3+, Cr3+, Ti4+ and/or In3+.
When using these starting materials, Mn4+-activated fluoride
phosphor compound with garnet-type crystal structure of other
compositions can be prepared as well.

[0053] An amount of the
{Na3}[A11.94Mn0.03Mg0.03](Li3)F12 powder
prepared as described above underwent further intense mechanically
grinding until the mean particle size was appr. 5 micron. Subsequently
the powder was pressed to a plate and sintered at 200° C. in a
furnace under an axial pressure of 2 kbar. After cooling to room
temperature, the so-obtained ceramic plate was scored with a laser and
broken into individual platelets. These platelets were used as wavelength
conversion elements in LED devices according to the present invention.

[0054] While the invention has been illustrated and described in detail in
the drawings and foregoing description, such illustration and description
are to be considered illustrative or exemplary and not restrictive; the
invention is not limited to the disclosed embodiments. For example, it is
possible to operate the invention in an embodiment wherein other
(optical) element(s) are present between the LED and the wavelength
converting elements or wherein more than one LED is operated in
combination with one converting element.

[0055] Other variations to the disclosed embodiments can be understood and
effected by those skilled in the art in practicing the claimed invention,
from a study of the drawings, the disclosure, and the appended claims. In
the claims, the word "comprising" does not exclude other elements or
steps, and the indefinite article "a" or "an" does not exclude a
plurality. The mere fact that certain measures are recited in mutually
different dependent claims does not indicate that a combination of these
measured cannot be used to advantage. Any reference signs in the claims
should not be construed as limiting the scope.